0

post: Creating a Schematic for an RFM69 Breakout Board
Moderators: adafruit_support_bill, adafruit

Please be positive and constructive with your questions and comments.

post: Creating a Schematic for an RFM69 Breakout Board

by sketchy on Tue Feb 25, 2014 12:07 pm

In this post I discuss how I went about designing the schematic and choosing components.
http://bitknitting.wordpress.com/2014/02/22/making-an-rfm69-breakout-board/
I want to start making PCBs. To get started, I designed a RFM69HW breakout board - similar in concept to the cc3000 breakout board but using the RFM69HW for wireless RF. I used kicad because this is what we're using in a course I am talking (Contextual Electronics).

I thought other folks that are "in the same boat" might be interested in what I learned along the way.

ALSO - I aspire to end up with a PCB populated with components that I have soldered on (seems intimidating right now...but...what the heck, I've got my rubber ducky in case I end up in the deep end...after all - just keep swimming!).

I would *VERY MUCH LOVE* to know how I can improve on the schematic. I also don't expect feedback - rather cherish it and learn from it when I receive it.

Thank you.

sketchy
 
Posts: 180
Joined: Thu Jun 28, 2012 11:37 pm

Re: post: Creating a Schematic for an RFM69 Breakout Board

by adafruit_support_mike on Tue Feb 25, 2014 5:11 pm

I see a minor problem with the way you've routed the decoupling capacitors for the two chips: you have them in series with the chip:

cap-series.jpg
cap-series.jpg (9.7 KiB) Viewed 994 times

and that won't work.

DC current can't flow through a capacitor. The symbol puts a gap in the 'wire' as a reminder of that.

Charging a capacitor is kind of like pushing one magnet with another.. voltage pushes charge carriers into one plate, and the increased charge on the plate pushes carriers with the same charge out of the opposite plate:

cap-charge.jpg
cap-charge.jpg (11.46 KiB) Viewed 994 times

The displacement makes it look like current is flowing through the capacitor for a little while, but a given voltage can only push so many carriers into one side and out the other. Once you reach that limit, the current apparently flowing through the capacitor stops. To push more charge in/out/through, you have to raise the voltage again. There's always a practical limit to how far your input voltage can rise, so there's always a limit to the amount of current you can send 'through' a capacitor.

The input voltage is the only thing keeping the extra charge carriers in the capacitor's upper plate, so if you reduce the input voltage, some of the carriers will flow back out of the upper plate. That allows carriers to flow back into the lower plate, creating the appearance of current flowing the capacitor the other way.

That's handy if you have a signal where you care about the changes (called the 'AC component'), but not about the absolute voltage (called the 'DC component'). It doesn't work for powering chips though.

A decoupling cap takes advantage of the in-and-back-out nature of charge on the upper plate, but to make it work you have to connect it like this:

cap-parallel.jpg
cap-parallel.jpg (9.59 KiB) Viewed 994 times

There's a DC path from the power supply to the chip's VCC pin, and also to the upper plate of the capacitor. The supply voltage pushes charge carriers into the cap just like before.

If the chip suddenly wants more than the DC path to the voltage supply can provide, the voltage at the chip's VCC pin will fall. That also lowers the voltage across the cap, so some of the charge carriers stored in the cap will flow out of the upper plate. That creates additional current for the chip's VCC pin.
When you void a product warranty, you give up your right to sue the manufacturer if something goes wrong and accept full responsibility for whatever happens next. And then you truly own the product.

adafruit_support_mike
 
Posts: 45725
Joined: Thu Feb 11, 2010 2:51 pm

Re: post: Creating a Schematic for an RFM69 Breakout Board

by sketchy on Tue Feb 25, 2014 6:00 pm

What a terrific response - thank you. Your explanation on decoupling capacitors is extremely useful. I'll update the blog post to include it. It does point out that - while I *thought* I understood capacitors, I really didn't. I am very grateful that you took the time you did to explain decoupling capacitors in a way that I find easy to grasp.

I have updated the schematic on github. https://github.com/BitKnitting/RFM69BreakoutBoard/blob/master/schematic.PNG

I ended up removing the extra decoupling capacitors (i.e.: those capacitors not near the voltage regulator) to simplify (similar schematics do not include the extra capacitors). There were several other fixes that extremely kind folks identified and I (hopefully) fixed.

I appreciate your response.

sketchy
 
Posts: 180
Joined: Thu Jun 28, 2012 11:37 pm

Re: post: Creating a Schematic for an RFM69 Breakout Board

by adafruit_support_mike on Wed Feb 26, 2014 1:07 am

sketchy wrote:It does point out that - while I *thought* I understood capacitors, I really didn't.

That's kind of a pattern in electronics.. you think you have things all figured out, then you discover a whole new layer of stuff you didn't know. Keeps it fun. ;-)
When you void a product warranty, you give up your right to sue the manufacturer if something goes wrong and accept full responsibility for whatever happens next. And then you truly own the product.

adafruit_support_mike
 
Posts: 45725
Joined: Thu Feb 11, 2010 2:51 pm

Re: post: Creating a Schematic for an RFM69 Breakout Board

by sketchy on Wed Feb 26, 2014 9:32 am

I've spent the last few hours trying to make sure I understood your explanation of decoupling capacitors.
Here's what I've come up with:
- in the "Name that thing" category (where naming is used to translate from some electronic activity to something easily understandable..but ultimately named based on history and/or a engineer's context…) a decoupling capacitor is 50% a good name. Why 50%? because a capacitor said to be decoupling can be tasked to do one or both of these jobs: 1) filter out high frequency noise (for this slather a .1uF capacitor - see figure 2 in this document: http://www.analog.com/static/imported-f ... MT-101.pdf). This is the decoupling aspect. Seems similar to me trying to filter out "NO YOU CAN'T" throughout my life… 2) provide charge when a component switches because the charge needed by the component come in bursts and the available charge might not have enough oomph.
-----
(WARNING: a conversational divergent you might wish to ignore)
The conversation above gives a great explanation of an IC using a capacitor as a gas station (can you tell I took home economics in school way back when but also worked at a gas station until some kids almost tore off my arm - i had to pump all cars at that point. Don't get me started about guys in the cadillac handing me a $1 and keeping their car running while their gas nozzle thingy was located strategically close to the yuck be emitted…but I diverge…sorry…).
-----

So then, I want to make sure the RFM69HW gets the energy it needs while filtering out the bad noise so that the circuit, the chip - heck our collective lives in learning!!! are the best they can be.

What capacitors should I put near the RFM69HW AND IN PARALLEL? :-)
1) a .1uF capacitor to filter out the high frequency noise (I'll refer to this as WHAT? OUCH TURN THAT SOUND OFF)
2) a 10uF capacitor to provide some of the charge/voltage the RFM69HW can use when it uses 130mA of current to transmit data. I pick this value because it seems many schematics use it…however…i am not clear if I were calculating the total amount needed to handle 130mA. Is using C=Q/V OK in this scenario? In that case C = 0.13A/3.3V = 0.04F … which seems large to me…which means I don't have an understanding of calculating the size of capacitor to use based on current used by the IC when the IC asks for a bump of current at high speeds…
I was looking at docs that explained this and perhaps if I spent more hours reading them slowly lights will dawn. I was hoping you could explain this far easier? Dunno...

Thank you again. How amazing it is to learn and apply this stuff!

sketchy
 
Posts: 180
Joined: Thu Jun 28, 2012 11:37 pm

Re: post: Creating a Schematic for an RFM69 Breakout Board

by sketchy on Wed Feb 26, 2014 12:17 pm

I was reading this (what I find very helpful) post on decoupling capacitors: http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html. It notes (important passage below) the larger capacitor for filtering out low frequency. So now it makes sense why a chip might have a larger capacitor along with a smaller capacitor close to it. But then is getting the charge "just" an added benefit - making the term "decoupling capacitor" a much better naming than I originally noted? (i.e.: the main function of a large and smaller capacitor circling in parallel near an ic is to filter low and high frequency. Given that a capacitor holds a charge, an added benefit that happens as the circuit switches is a close source of current that makes it faster for the state of the ic to change)

For effective supply de-coupling that frequency has to cover as wide a range as possible and certainly the range of noise being generated by the circuits. So this is why we often see two different values of capacitor used in parallel. One large one to filter out the low frequency variations and a small on for the high frequency noise. Typical values are 47uF and 0.1uF or 0.01uF (10nF) and as low frequency variations are system wide you tend to need only one large capacitor. However, high frequency noise is no so easily suppressed and so you tend to need a small capacitor on every, or every other chip.

sketchy
 
Posts: 180
Joined: Thu Jun 28, 2012 11:37 pm

Re: post: Creating a Schematic for an RFM69 Breakout Board

by 1chicagodave on Wed Feb 26, 2014 4:57 pm

If a picture is worth 1000 words....how about a moving picture?
At least for me, sometimes a few well-made animated GIFs work great for getting a new concept to finally 'click'. (Also a great reason to own a decent oscilloscope!)

Here are a few about decoupling & bypass caps -
http://www.williamson-labs.com/480_rlc-c.htm

On same site, focusing on bypass caps -
http://www.williamson-labs.com/480_byp.htm
Please forgive my hyperanthropomorphization.
Every answer reveals new questions.
And, yes... there's always a better way to 'do that'.
User avatar
1chicagodave
 
Posts: 564
Joined: Wed Jun 19, 2013 3:35 am
Location: Chicago

Please be positive and constructive with your questions and comments.